The invention relates to a method and apparatus for evaluating the frequency or wavelength properties of a light beam and, more particularly, to a method and apparatus for determining whether a laser beam has frequency properties meeting predetermined criteria through the use of a fringe pattern of alternating light and dark bands which represent particular frequency properties of the beam.
The utilization of laser light in various fields of technology is becoming more and more widespread. In many laser applications, it is necessary to know the makeup of the laser light beam in terms of the bandwidth of radiation which constitutes the major portion of light beam. This makeup is usually referred to as the spectral content of the light beam and is often defined in terms of the range of frequencies or wavelengths of radiation contained in the beam.
Ideally, monochromatic light consists of electromagnetic radiation of a single frequency .nu. or wavelength .lambda.. In practical applications, however, monochromatic radiation is characterized by a center frequency .nu..sub.o and a bandwidth .DELTA..nu. such that a frequency interval .nu..sub.o -.DELTA..nu./2 to .nu..sub.o +.DELTA..nu./2 contains a large part of the energy of the radiation. It is the extent of this range or interval of frequencies which is of particular interest in the application of laser light. For example, this information is useful in the field of holography where light composed of radiation falling within a maximum allowable frequency range is necessary for proper imaging. Instruments which are responsive to laser light, such as optical countermeasure receivers, may require substantially monochromatic light of a narrow bandwidth as well.
A common method of expressing the spectral content of a light beam is in terms of its coherence length. The coherence length L.sub.c may be defined as L.sub.c =c/.DELTA..nu., where c is the speed of light and .DELTA..nu. is known as the temporal bandwidth. Since the coherence length is inversely proportional to bandwidth, it can be seen that for applications of laser light such as those discussed previously, it is desirable that the light beam have a maximum attainable coherence length. Light having at least a predetermined minimum coherence length is typically referred to as "coherent light", i.e. light comprised of electromagnetic radiation with a major portion of the radiation energy falling within a predetermined relatively narrow bandwidth of frequencies or wavelengths.
At present, the only successful method of measuring the coherence length of a light beam is through the use of multiple beam interferometers. An optical device such as a Fabry-Perot etalon breaks up a beam of light into a number of beams which interfere with one another to create a fringe pattern indicative of the spectral content as a function of radiation wavelength or frequency of the beam, the angle of incidence of the beam on the etalon and the distance along the face of the etalon from a predetermined edge thereof. The fringe pattern is analyzed to determine the coherence length of the light beam.
When this analysis is to be done electronically with light detectors, complex and costly circuitry is required in accordance with prior devices. For example, in U.S. Pat. No. 3,824,018 to Crane a frequency discriminator, among other components, is called for. Also shown in this patent, as is typical of most prior art systems, is the need for an etalon having relatively low finesse.
Finesse is a property of an interferometer which is determined by the reflectivity of the reflecting surfaces of the interferometer. It is a measure of the width of a band of light in a fringe pattern in relation to its distance from the light band of an adjacent order for monochromatic light. Use of a low finesse etalon results in a fringe pattern in which the light and dark bands are less sharply defined. This limits the degree of resolution which can be obtained in the analysis of the fringe pattern.
Many of the prior art coherence length measurement systems utilize a plurality of etalons with each etalon having a light detector associated therewith. Such systems present manufacturing problems because only extremely low tolerances are acceptable in the design of the etalons in order to obtain the small thickness differential between etalons which is necessary to achieve a valid coherence length measurement.
Design problems are also encountered in the prior art systems. A number of design variables such as the coherence length threshold (i.e. the minimum coherence length that is useful in a particular application), the spectral range of measurement for which the system is designed, the number of light detectors used and the finesse of the etalon are interactive and cannot be independently controlled, resulting in unwanted compromises in measurement and extreme complexity.